Suggestion: Add example of or link to multilevel splitting code

[jmgirard]

Section 5.3 makes a point that it is important to split the multilevel data by the highest level experimental unit (e.g., participant rather than observations of participants). However, it doesn't provide a clear way to do this in the tidymodels ecosystem. I recommend adding either a quick example chunk below that show how to do this or a link to a vignette or wiki page or something that shows it.

[juliasilge]

This is related to tidymodels/rsample#207, where you'll see that really fluent splitting by group isn't implemented yet. Folks have to do this somewhat manually as of today, as shown in links there. I don't think we'll get this implemented by publication so we can update the website afterwards.